1. Field of the Invention
The present invention relates to a leaky coaxial cable.
2. Description of the Related Art
A leaky coaxial cable (LCX) is such that a plurality of slots are provided as a radiating part on an outer conductor of an ordinary coaxial cable. An electromagnetic wave signal supplied to an inner conductor may be shielded by the outer conductor, but leaked outside through the slots serving as the radiating part. More specifically, through the slots, the electromagnetic wave signal in the cable may be radiated outwards, or the electromagnetic wave signal outside the cable may be taken into the cable. In other words, the LCX may be a cable type antenna and a specialized, long and thin transmitting and receiving antenna.
The LCX is widely used as a communication line for a moving vehicle, such as a railroad, a car and the like. In an application to a wireless communication of a train, the LCX which is laid along a railroad line can serve as a communication antenna with an antenna provided in a railroad vehicle. Also, in recent years, the LCX can be used as an antenna for a wireless LAN.
In the conventional LCX, a metal tape having slots formed by a punching process is used as the outer conductor (refer to T. Kishimoto and S. Sasaki, “LCX Communication System”, The Institute of Electronics, Information and Communication Engineers, Aug. 20, 1982 (S57)). In this case, since one lengthwise metal tape is added in a longitudinal direction of the LCX, there is a problem of inferior flexibility. Also, because of inferior flexibility, a crack may be generated in the outer conductor from the slots when the LCX is bent.
In order to produce the LCX having superior flexibility, an idea of using the outer conductor of a braided wrap type or a serving (or spiral) wrap type, which is spirally wrapped around the insulator, is proposed (refer to Japanese Patent Laid-Open No. Hei 9(1997)-198941 and Japanese Patent Laid-Open No. 2003-123555). Gaps between adjacent outer conductors can be used as the radiating parts. In the proposed outer conductor, since the braided wrap or the serving wrap of wires, or the metal tape is used, flexibility can be improved.
However, since the spirally wrapped outer conductor is used, design freedom of the pitch of the radiating part may be degraded. It is actually difficult to make the angle of the braided wrap or the serving wrap to approximately 10 degrees or less, and thus there is a limit to increase the pitch of the radiating part . For example, in a case that an outer diameter of an insulator is about 5 mm, the limit of the pitch of the radiating part may be about 90 mm or less. Also, in the conventional LCX, since the pitch of the radiating part corresponds with a signal wavelength at a frequency where a radiation angle is vertical to the axis direction of the LCX, a large voltage standing wave ratio (VSWR) is generated in the LCX, and such LCX may be useless.